a. Field of the Invention
This invention relates to a cord spool for a cord driven mechanism for controlling the retraction and deployment of an architectural covering, especially a covering for an architectural opening, such as a window blind or shade. This invention particularly relates to a cord spool which counteracts the increasing downward force which must be applied to an operating cord to open a window covering--to overcome the increasing weight and/or frictional resistance of the covering, as it is opened. The invention quite particularly relates to a cord spool for a venetian blind which is raised by winding lift cords around a central shaft or on separate spools on the central shaft of the head rail of the blind.
b. Description of the Related Art
When raising a conventional Venetian blind to open it, its lift cords pull its bottom rail upwardly as a result of its operating cord or pull cord being pulled downwardly or of other means, such as a motor. As the percentage of the blind that is lifted increases, the weight of the blind, already lifted, pulls downwardly on the lift cords. The percentage of the blind that has not yet been lifted is supported by its ladder cords.) The force needed to continue to raise the blind has to overcome continuously the increasing weight of the amount of the blind already lifted. In this regard, the initial raising force overcomes the weight of the bottom rail. Subsequent raising forces are variable and are related to the percentage of the blind, already raised. This is because the weight, to be pulled up, increases as the percentage of the blind already raised increases. Additionally, friction in the cord system for raising the blind will also gradually increase as the blind is raised, thereby increasing the required raising forces.
Thus, there will normally be a significant gradient in the raising forces needed to open a Venetian blind. If a person is raising the blind by pulling downwardly on its operating cord, the person will experience the significant raising force gradient as the blind is opened. If the person is strong, he will often use excessive force to raise the last part of the blind. This can cause the bottom rail of the blind to slam into the head rail, thereby damaging the blind. Excessive raising forces pulling on the operating cord can also make it wear more quickly. By comparison, people with muscle-weakening diseases or rheumatism will often find it difficult to fully raise the blind comfortably because of the significant raising force gradient. If a motor is used, the motor will either be adapted to supply a constant force, which will raise the last part of the blind more slowly, or to supply extra force to compensate for the significant raising force gradient encountered in raising the blind. This often requires expensive electronic circuitry and/or elaborate automatic gear boxes in motor driven blinds.
Cord spools, which counteract the increasing raising forces needed to open such blinds, are known, for example from U.S. Pat. No. 13 251 (1855) and French patents 1 157 647 and 389 516. In such cord spools, an operating cord is wound around a threaded cone. As a blind is raised, the operating cord is wound around progressively larger diameters of the cone. The leverage effect of the larger diameters increases the raising forces to counteract the increasing weight of the percentage of the blind, already raised.
Such cord spools have several drawbacks. One drawback is that all the operating cord is stored on the cord spool, which takes up a lot of space. Another drawback is that there is no relation between the increasing weight of the percentage of the blind, already raised, and the increased raising force provided by the leverage effect caused by winding the operating cord on increasingly larger cone diameters. As a result, the increase in raising force can easily exceed the increasing weight of the percentage of the blind, already raised, resulting in excessive raising forces being exerted on the last part of the blind, to be opened. This can cause the bottom rail of the blind to slam into the head rail. The reason for this problem is that it is very difficult to correctly dimension such a cord spool in relation to the total weight of a blind. In this regard, the correct dimensions of the cord spool (i.e., its length, slope, and thread pitch) and the length of the operating cord would have to be calculated against the height, width, number of slats and total weight of the raised blind. These calculations would have to be made for each blind, since blinds are often made to measure. This would require, in effect, a differently dimensioned cord spool for each different blind which is highly undesirable in modern manufacturing where head rail mechanisms are typically designed for a wide variety of sizes and types of blinds.